Airfield runway lighting exemplifies an application of the communications system of this patent. Airfield lighting systems use lamps positioned along runways, taxiways and runway thresholds to aid airmen in aircraft operation to, from, and on an airfield, particularly at night or in adverse weather conditions. Associated groups of lamps, for example those lining one or both sides of a runway, are typically energized from a single energization line or power loop circuit. The lamps themselves are typically low wattage incandescent bulbs.
Each lamp of a group is located, typically one to a circuit, in a secondary circuit to isolate it from the power loop in case the lamp burns out or is missing. The secondary circuits which contain the lamps are electrically coupled to the power loop by isolating transformers. The primary windings of the isolating transformers are connected in series between the input and output of the power loop and the secondary windings are connected directly to the lamps.
The primary power loop is supplied with energy by a regulated AC current source, which provides an alternating current with a constant root mean square current amplitude. Upon failure of a lamp, the secondary circuit associated with an isolating transformer will be in an open circuit condition, with a consequent effect upon the load it imposes on power loop during its operation. Such changes in load have subsequent measurable effects on the power signal.
A number of prior art systems exploited such phenomenon to generate indication of the existence and number of lamp failures in a group. However, such systems generally could not identify particular failed lamps. Identification of the location of a failed lamp still required personal inspection of all lamps in a circuit to locate a failed lamp. Such personal inspection can prove a complicated job on large airfield, and a tedious, time consuming procedure at any airfield.
However, the information collected by such inspection, once existence of a failure is determined, is quantitatively small, i.e. the locations of the failed lamps. A more complete data set for the lamps of a group would include the on/off status of each lamp by location, which is still, quantitatively, a small amount of information. Notwithstanding the minimal amount of information to be collected, the low density of the data sources compared to the length of cable required to collect the information has made it uneconomic to use conventional data transmission systems for collection of the information.
The problem of collecting the information from airfield lighting systems is qualitatively similar to collecting electrical power meter readings from residences and other commercial power users. Here the problem again includes widely distributed, but numerous data sources, each generating data at an extremely low rate. Notwithstanding a direct physical connection to electrical power meters, utilities still commonly employ personal inspection of meters to determine power usage.